Shape memory alloys hold promise in the building and construction industry, but first they must meet engineering requirements. Researchers in Switzerland have developed new iron-based alloys that could potentially be produced on a mass scale.
Shape memory alloys, or SMAs, possess the ability to return to their original shapes after being severely deformed, either spontaneously or following the application of heat. This makes them useful materials, not just for making spectacle frames but also for technical applications such as thermostats, stents, and micro-actuators. Applications in the construction industry are conceivable, too, for example, in the reinforcement of bridges.
If a concrete beam is cast with reinforcing rods made of an SMA material, these can then be “activated” through the application of heat. They attempt to return to their original shapes, but because of their concrete sheath, cannot do so, thus exerting a pre-stressing force on the beam.
This effect can be used, for example, to pre-stress a complete bridge span. In order to generate the necessary force, the SMA rods must simply be heated by passing an electric current through them. This obviates the need for using elaborate tensioning systems and jacket tubes, as used in conventional pre-stressing techniques.
The nickel titanium alloys used to make spectacle frames or stents are not very suitable for use in the construction industry. Iron-based SMA products are much more attractive, since both the raw materials and the processing costs are far cheaper. However, to date, one problem has remained a stumbling block: to activate the memory effect, the materials currently used must be heated up to 400°C, which, for applications involving concrete or mortar or other heat-sensitive materials, is too high.
At the Swiss Federal Laboratories for Material Science and Technology, also known as Empa, researchers, led by Christian Leinenbach of the Laboratory for Joining Technologies and Corrosion, have succeeded in developing a novel iron-manganese-silicon SMA alloy that is activated at just 160°C, a temperature much more suitable for use with concrete.
The material science researchers “designed” a range of virtual alloys using thermodynamic simulations and then selected the most promising combinations. These were then manufactured in the laboratory, and their shape memory characteristics were tested with great success. Several of the new materials met construction engineering requirements, an important milestone on the path to providing economic shape memory steel alloys for industrial applications — in other words, manufacturing them by the ton.
The Long Road from Lab to Finished Product
Christoph Czaderski, of Empa’s Structural Engineering Research Laboratory, believes that iron-based SMA materials have a promising future in the building industry, since the process of pre-stressing is simpler and therefore cheaper than conventional techniques. In addition, they may allow engineers to create pre-stressed structures, which are impossible or very difficult to achieve using conventional techniques.
These include the use of short-fiber concrete, near-surface-mounted laminates, column wrapping, and ribbed armoring steel. A feasibility study financed by Switzerland’s Commission for Technology and Innovation (CTI) recently showed that it is possible to produce the new alloys on an industrial scale, not just a few kilos for laboratory use. The manufacturing process has been developed in collaboration with the University of Leoben (Austria), Technical University Bergakademie Freiberg (Germany), and the German company G.RAU GmbH.
The working of cast ingots — each about 100 kg in weight — into thin strips around 2 mm thick or ribbed armoring steel rods at temperatures over 1000°C calls for high degree of technical knowledge and the appropriate infrastructure. The working process also needs to be adapted for use with the novel alloys.
To carry forward the developments made at Empa, a start-up company, re-Fer AG, has been set up. It will produce and distribute iron-based SMAs for the construction industry. The cost of the new products is expected to be about the same order of magnitude as that for stainless steel-based materials.
Top photo credit: Empa